This invention generally relates to material tests in hot water and more particularly, to a reference electrode or pH probe for use in material tests in hot water.
This invention relates to hydrogen ion, or pH, sensors. Such sensors are employed to determine the content of a specific substance in a fluid or atmosphere. For example, a sensor can be employed to determine the content of oxygen, or carbon dioxide, or the content of hydrogen ions or other ions in solution.
Hydrogen ion sensors are normally utilized for measuring ion concentrations of liquids at approximately ambient temperatures or in some cases, at temperatures ranging up to 150xc2x0 F. However, innovations such as geothermal wells, nuclear reactors, and the use of other high temperature fluid systems has created a need for a probe that can measure the pH of aqueous fluids at a temperature of 300xc2x0 F. or higher.
Further, because the pH at certain points in these systems can be of critical importance, there is a need for a pH probe with the flexibility to allow the electrode to be placed within the system precisely where the fluid pH is of most interest. The current state of the art includes the following references: L. W. Niedrach, Application of Zirconia Membranes as High-Temperature pH Sensors, General Electric Technical Information Series, No. 83CR0147 (June, 1983); Leonard W. Niedrach, A New Membrane-Type pH Sensor for Use in High Temperature-High Pressure Water, J. Electrochemical Soc., Vol. 127 No. 10, p. 2122 (October, 1980); U.S. Pat. No. 4,264,424 to Niedrach; U.S. Pat. No. 4,406,766 to McDonald; and U.S. Pat. No. 4,575,410 to Neti.
The Niedrach-type standard high-temperature pH probe consists, essentially, of a zirconia ceramic tube or sheath surrounding both a metal wire and some form of oxide powder inside of the tube. This tube is supported in a compression fitting, with the fitting forming a pressure boundary seal around the zirconia tube. The potential produced between the internal wire of the electrode and a reference electrode outside of the probe is proportional to the pH of the solution outside the tube. Because the electrode of the Niedrach references uses a gas impervious membrane tube or sheath of an oxygen ion conducting ceramic, and because such ceramics are rigid and brittle, this electrode cannot be bent around corners, or located in restricted regions to measure local pH.
The electrode device of the MacDonald patent is a dual electrode device and includes a chamber to equilibrate pressure (to minimize electrolyte dilution). Ag/AgCl is used as a reference reaction, and the reference reaction is at room temperature. The electrode of the MacDonald patent is not flexible, being a ceramic tube, making the electrode too rigid for some high-temperature applications.
The electrode device of the Neti patent is also a dual electrode device and uses metal/metal halide reference reactions at elevated temperature for the pH sensing electrode. The electrode is usable at high temperatures and pressures, but is not flexible and is too rigid for some applications. A reference electrode is disclosed which can be made of polytetrafluoroethylene, among other materials.
The reliability and accuracy of existing reference electrodes and pH probes at high temperatures is less than optimal. More specifically, existing pH probes are fragile, rigid and difficult to use in various high-temperature applications. In general, prior art pH probes fall into two categories, those which are flexible but cannot perform at temperatures sufficiently high for some applications, and those which sacrifice flexibility and thus are limited in the placement of the probe tip in high-temperature measurements.
In summary, there is currently a need in the art for a pH probe which is flexible enough to enable use thereof in applications that require flexibility, yet which is able to withstand very high temperatures.
It is, therefore, an object of the invention to provide a pH probe that can function in high-temperature aqueous environments.
It is a further object of the invention to provide a pH probe that is more flexible than conventional high-temperature pH probes.
It is another object of the invention to provide a pH probe that is more durable than conventional high-temperature pH probes.
It is yet another object of the invention to provide a pH probe flexible enough to allow placement of the electrode at virtually any specific point of interest.
As will become apparent, the key advantages of the invention include improved flexibility and reduced fragility, and simplified construction and utilization. The flexibility provided enables placement of the electrode into the area of interest, including around corners, into confined areas and the like. Accordingly, in general, the invention provides a substantially improved pH probe for use in hot water and other relatively high temperature environments, at temperatures above 300xc2x0 F., and up to about 590xc2x0 F., and represents a major improvement in the technology.
In accordance with the invention, there is provided a pH probe device comprising: a flexible, inert tubular probe member having a distal end; an oxygen anion conducting, solid state electrolyte plug located at the distal end of the tubular member; oxide powder disposed at the distal end of the tubular member; a metal wire extending along the tubular member and having a distal end in contact with the oxide powder so as to form therewith an internal reference electrode; and a compression fitting forming a pressure boundary seal around a portion of said tubular member remote from the distal end of the tubular member.
Preferably, the tubular member comprises a polytetrafluoroethylene tube.
The oxygen anion conducting, solid state electrolyte plug preferably comprises a zirconia plug and more preferably, a stabilized zirconia plug. Advantageously, the metal of the metal wire and the oxide of the oxide powder of the internal reference electrode are a metal/oxide selected from the group consisting of copper/copper oxide (Cu/CuO), iron/iron oxide (Fe/Fe3O4), silver/silver oxide (Ag/Ag2O), nickel/nickel oxide (Ni/NiO) and mercury/mercuric oxide (Hg/HgO).
In a preferred implementation, the oxide powder is contained in a chamber located at the distal end of said tubular member and disposed proximally of the plug such that the plug holds said powder within said chamber. Advantageously, the probe further comprises a heat shrink covering holding the plug in place at the distal end of the tubular member. Advantageously, the heat shrink covering comprises polytetrafluoroethylene.
Further features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.